Photo-electric tube



June 935- .71. H. DE BOER El AL 2,003,344

PHOTO-ELECTRIC TUBE Filed May 15, 1930 INVENTORS JAN HENDRlKdeBOER BYMARTEN coaneus waves 7AT T(5R EY Patented June 4, 1935 PHOTO-ELECTRICTUBE Jan Hendrik de Boer and Marten Cornelia Teves, Eindhoven,Netherlands, assimora to Radio Corporation of America, a corporation ofDelaware Application May 15. 1330, Serial No. 452,785

, In the Netherlands may 16, 1929 11 Claims.. (Cl. 250-275) Theinvention relates to photoelectric tubes,

ing particles may be applied to the substratum and more particularly tothat type of photoelecby simultaneously evaporating this compound andtric tubes which comprises an electrode containing a photo-activesubstance which on being subjected to radiation emits electrons, thenumber of which depends on the intensity of the radiation.

The applicants already have previously proposed to provide between thephoto-active substance and a supporting surface or substratum carryingthe photo-active electrode, an intermediate layer containing a chemicalcompound which is preferably so chosen that the photoactive substance isbetter adsorbed by the intermediate layer than by the substratum. Thechemical compound may advantageously consist of an oxide or a halide,.for example of calcium fluoride. Now the applicants have found that thevalue of the electric resistance of such a photo-active electrode may bevery high, which is not favorable for the sensitiveness of the tube.Theinvention has for its object to obviate this disadvantage and todecrease the resistance of photo-active electrode.

In a photoelectric tube according to the in--- vention, the photo-activesubstance, constituted, for example, by cesium, is provided on anintermediate layer which contains a chemical compound and electricallyconducting particles. The electric resistance of such an electrode isconsiderably smaller than that of an electrode in which the intermediatelayer has no conducting particles. Conveniently, these particles may beconstituted by particles of the photo-active substance which for thispurpose may becaused to penetrate into the intermediate layer owing towhich at the same time the emission of electrons is increased. Thisseems to result therefrom that electrons arealso liberated in theelectrode by the light falling through it.

The penetration of the photo-active substance into the intermediatelayer may be effected by heating the tube which has been closed afterphoto-active material has been provided on the intermediate layer. Thepenetration of the photoactive substance into the intermediate layermaybe facilitated by causing this layer to contain particles of anotherconducting material. Besides, the resistance of the photo-activeelectrode may be diminished furthermore by these conductive particles.

The intermediate layer utilized in accordance with the invention may beformed in difierent ways. The chemical compound and the conductthe aconducting material. These substances may be provided in the tube, forexample on an electrode. The intermediate layer should preferably beformed by, a chemical reaction, in which event the reactivesubsta'ncesmay be provided, by evaporation, on the substratum. In thiscase the reaction may advantageously be started with a difflcultlyvolatile metal which is heated in the tube, preferably in a vacuum,together with a chemical compound of -a readily volatile metal and anegative constituent capable of forming with the difficultly volatilemetal a readily volatile compound. In this case, the starting materialsto 5 be utilized are so chosen that the evaporating substances reactwith each other so that the starting materials are formed again, atleast partly, on the substratum in a finely divided and thoroughly mixedstate. To the mixture, which is to be heated in the tube, may also beadded another substance ,which facilitates the chemical reaction orpartly produces other products.

It is also possible to provide the chemical compound present in theintermediate layer and the photo-active substance, and if desired alsoother conducting particles, simultaneously on thesubstratum. For thispurpose thephoto-active substance may be advantageously developed inthetube or in a space connected thereto by heating a mixture containinga compound of the photoactive substance, for example cesium chromate,and a reducing agent, for example, zirconium. In addition, the mixturemay contain a chemical compound, for example, sodium-fluoride, whichevaporates during the heating or reacts with the reducing agentin themanner described. in the preceding paragraph. a

The invention will be described more fully with reference to theaccompanying drawing which represents diagrammatically, by way ofexample, a photoelectric tube according to the inven tion. Inthedrawing:

Figure 1 is a section of a photoelectric and Figure 2 shows a detailthereof.

The photoelectric tube illustrated has a wall I consisting, for example,of glass, to which is applied a layer 2 of photo-active material, forex- 50 ample oi" alkalimetal. However, instead of being applied directlyto the wall of the tube, the layer tube;

' 2 isapplied to an intermediate layer 3 which contains a chemicalcompound, for example, calcium fluoride. and conducting particles. The

photo-active electrode is connected to a supply wire 4 which is takenout through the wall of the tube. On the stem 5 of the photoelectrictube is arranged an electrode 6, which, as distinctly appears fromFigure 2, has the shape of a nearly closed ring and which. is connectedto supply wires I and 8.

A .method is hereinafter described, with the aid of which theintermediate layer 3 can be formed and the conducting particles may beprovided in the intermediate layer so as to have a finely divided formand to be thoroughly mixed with the chemical compound. Prior to thearrangement of the electrode 6 in the tube, this electrode is coatedwith a mixture containing a difiicultly volatile metal and a compound ofa readily volatile metal with another ingredient capable of forming withthe diflicultly volatile metal a readily volatile compound. The mixturemay contain, for example, tungsten and calcium pass through theelectrode 5. Due to this heating,

the substances present in the mixture react with each other. Forexample, the tungsten reacts with the calcium fluoride so as to formtungsten fluoride and calcium. This reaction might be represented by thefollowing equation:

The produced volatile compound of the difllcultly volatile metal and thereadily volatile metal present in the mixture (in the present case,tungsten fluoride and calcium) evaporate oif the electrod. Inconsequence thereof. the reaction, which is reversible, will take placeas indicated by the equation. The evaporated substances are deposited onthe wall of the tube and react again with each other owing to which thestarting materials may be formed again. The reaction taking place inthis case might be represented by the following equation:

Owing to this process, the conducting particles (the tungsten particles)and the chemical compound (in the present case the calcium fluoride) arepresent in the intermediate layer in a very finely divided andthoroughly mixed state, which is very favorable for the functioning ofthe photoelectric tube. Other diflicultly volatile metals which may besuccessfully utilized are, for example, iron and zirconium, while thecompound of the easily volatile metal which is'heated together with thedifllcultly volatile metal may also consist. for example, of sodiumchloride or potassium chloride.

In many cases it may be advantageous to add to the mixture heated in thetube another substance which facilitates the reaction or which producespartly other products. Thus, for example, silica may be added to themixture containing tungsten and'calcium fluoride. The reactions in thiscase might have the following course:

During the heating of the mixture, the tube is connected to the vacuumpu p whereby the very volatile silico-fluoride is removed from the tube.

The tungsten oxide and the calcium evaporate and settle on the wall ofthe tube. These substances react with each other which might berepresented by the following equation:

The intermediate layer thus produced consists connected at 9 to thetube. A suitable method of introducing alkali metals into the tube isdescribed, for example, in application for U. S. Letters Patent, SerialNo. 360,993, flled' May 6, 1929.

After the photo-active substance is applied to the-intermediate layer,the tube may be closed and heated. If the photo-active substance isconstituted by cesium, the tube may be heated, for example, to about 200C. Due to the heating of the closed tube, the photo-active substancepenetrates into the intermediate layer, thus giving rise to a decreasedresistance of the photoactive electrode and to an increased emission ofelectrons. Not only the photo-active substance, which at the beginningof the heating operation is present on the intermediate layer, partlypenetrates into the latter, but also the photo-active substance presentin other points of the tube tries to penetrate into this layer due tothe fact that during the heating of the tube it evaporates and depositson the intermediate layer. Preferably, the quantity of the photo-activesubstance provided in the tube is such that after'the closed tube hasbeen heated all the photo-active substance present in the tube ispresent in the intermediate layer or is adsorbed by this layer so thatin the tube the vapour pressure of the photo-active materialis smallerthan the normal pressure of the free material. Any superfluousphoto-active material present in the tube may be removed therefrom byheating the tube while connected to the vacuum pump.

It is evident that the process in which a closed tube containing thephoto-active material is heated to cause this material to penetrate intothe intermediate layer can also be applied to a tube in which thephoto-active substance is provided on an intermediate layer which doesnot yet contain conducting particles but which exclusively consists of apoorly conducting chemical compound. However, applicant hes found thatthe penetration of the photo-active substance into the intermediatelayer is facilitated if the latter already contains conductingparticles.

It is also possible to apply the chemical compound present in theintermediate layer, and the photo-active substance, and, if desired,also other.

example, zirconium. If the mixture contains, in addition, a chemicalcompound which is volatile at the temperature to which the mixture isheated, the liberated photo-active metal and the said chemical compoundprecipitatesimultaneously on the substratum.

To the mixture of the compound, of the photo- .active metal and thereducing agent may also be added a compound of a readily Volatile metal,for example, sodium fluoride, which reacts with the reducing agent inthe same manner as above described for tungsten and calcium fluoride. Ifthe mixture consists, for example, of cesium chromate, zirconium andsodium fluoride, cesium will be liberated by some of the zirconium fromthe cesium chromate leaving oxides of chromium and zirconium and theremainder of the zirconium reacts with the sodium-fluoride to formzirconium-iluoride and sodium, which evaporate and, reacting in much thesame manner as tungsten fluoride and calcium produce on the substratumagain zirconium and sodium fluoride.-

The layer formed on the substratum consequently contains zirconium,sodium fluoride and cesium in a finely divided and thoroughly mixedstate while at the surface of this layer a thin layer of cesium will beformed.

During the formation of the intermediate layer and the introduction ofthe photo-active substance into the tube, those parts of the wall'of thetube on which precipitation is undesirable, may be maintained at ahigher temperature than theother parts of the wall, so that a window l0may be formed through which during the operation of thetube the rays oflight are enabled to enter into thelatter.

The window I0 may also be produced by screening with the aid of a shielda part of the wall of the tube during the formation of the intermediatelayer and the introduction of the photoactive substance into the tube.

The tube, which is provided with an exhaust tube ll allowing ofconnecting it to the pump, may be exhausted to the highest possibledegree or may be filled with a gas, for example, a rare 888.

During the operation of the photo-electric tube represented, theelectrode 6 is given a positive potential with relation to thephoto-active electrode, which is subjected. to radiation by light whichcan enter the tube through the window Ill and which causes the emissionof electrons from the photo-active layer, the number of the electronsemitted depending upon the intensity of the light. It has been foundthat the photoactive electrode of the tube according to the inventionmay show a high emission of electrons while the electric resistance ofthis electrode is particleson the supporting surface, and a photo-'active substance coated upon said intermediate layer.

ried upon the intermediate layer.

3. A photoelectric cathode comprising'a sup- 2. A photoelectric cathodecomprising a supporting surface, an intermediate layer containing achemical compound and conducting particles of a photo-active substancecoated upon the supporting surface, and a photoelectric layer carportingbase, a layer of. photo-active substance, and an intermediate layercontaining a chemical compound and photo-active electrically conductingparticles.

4. A photoelectric cathode comprising a supporting base, a. layer ofphoto-active substance,

and an intermediate layer containing a chemical compound and metallicelectrically conducting particles.

5. A photoelectric cathode comprising a supporting base, a layer ofphoto-active substance,

and an intermediate layer containing a chemical compound and thecombination of photo-active plying photoelectric material upon thedeposited,

layer.

8. In the process of forming photoelectric tubes, the steps comprisingdepositing alayer of a mixture of electrically conducting particles anda .salt by double decomposition upon a substratum,

and applying photoelectric material upon th deposited layer.

- 9. In the process of forming photoelectric tubes, the steps comprisingdepositing by evaporation and forming by double decomposition a layer ofa mixture of electrically conducting particles and -a salt,.and applyingphotoelectric material .upon

the deposited layer.

10. In the process of forming a. photoelectric tube, the steps whichcomprise heating a. mixture of cesium chromate and zirconium containingalso a volatile, constituent capable of reacting with a. reducing agentupon a base layer, and applying photoelectric material upon thedeposited layer. i

11. The process of preparing a photo-electric cathode whichcomprisesplacing a mixture of a refractory metal and a compound of a.readily volatile metal and an ultimately inactive element in anevacuated photo-envelope, heating said mixture to form a volatilecompound of said refractory metal, exhausting said volatile compoundformed from said envelope and, reacting the residual substances in vaporphase to reform one of the original materials and a diiferent metalcompound of another original metal compound as, a finely divided andthoroughly mixed layer on the inner wall of said envelope, and finallyvaporizing photo-electric material to be absorbed by said mixed layer.

' JAN HENDRIX 1m BOER.

MARTEN CORNELIS TEVES.

